The invention relates to devices for reading the amounts of electrical charges borne by a dielectric sheet, this term "sheet" denoting both, in the following, a sheet proper and an analogous support such as a plate, a drum or a layer coating a flat or cylindrical surface.
These devices enable the "reading" of data or images recorded electrically on such sheets.
By the expression "reading" is meant in the present description both reading proper manifested by the display of an indication on a suitable apparatus, with or without recording and the determination of an immediately employed magnitude, without special display, particularly for recording purposes, for automatic correction, for warning or the like.
The invention is aimed more particularly, among reading devices of the type concerned, at those which comprise a first electrode positioned facing the surface of the sheet bearing charges to be read, said electrode being called "probe" in the following, a second electrode positioned against the surface, of the sheet, opposite the probe, means for passing the sheet opposite the probe or inversely, and electronic means to measure at each moment the quantities of the image charges induced on the probe by the quantities of charges to be read.
It is aimed more particularly also at electronic means, for measuring image charges, comprised by the reading devices concerned.
In known modes of construction, these electronic means were designed to carry out measurements on the potentials of the image charges concerned.
These electronic means presented certain drawbacks.
In particular they required very high input impedances, which rendered these devices:
of poor stability over time in view of the inevitable progressive degradation of the high impedance components as a result of humidity and other sources of pollution, which generate leakage paths, PA1 sensitive to electromagnetic interferences, high impedances playing the role of an antenna with respect to them, PA1 and, especially, relatively inaccurate due to the fact of the low value of the signal/noise ratio. PA1 an input amplifier provided and mounted so that, on the one hand, it transforms the current I.sub.o applied to one of its inputs into amplified output voltages V.sub.1 =K I.sub.o, in which formula K denotes a coefficient having the dimensions of an impedance, and that, on the other hand, its input potential, which is that of the probe, remains permanently as close as possible to the reference potential, generally nil, to which the second above electrode is brought, PA1 and an analog integrator of the voltage V.sub.1 associated with chronic reset to zero means actuated at moments when the probe is positioned facing a zone, of the sheet, brought to reference potential. PA1 the input amplifier is an operational transimpedance amplifier, PA1 the relative movement between the probe and the sheet to be read is effected by line-scanning and the chronic reset to zero of the integrator is effected automatically at least once on the scanning of each line, particularly at the start of this scanning, the probe then passing opposite a line borne by the sheet and subject to the reference potential.